Avoid reparsing keyframe selectors

Source/core/css/CSSGrammar.y.in

Index: Source/core/css/CSSGrammar.y.in
diff --git a/Source/core/css/CSSGrammar.y.in b/Source/core/css/CSSGrammar.y.in
index 9543cbead6f9f1da23567ec934cb740a2cb4b194..5ee92d3e7abbd07a9a22b3c42d2eeb373315e78e 100644
--- a/Source/core/css/CSSGrammar.y.in
+++ b/Source/core/css/CSSGrammar.y.in
@@ -847,7 +847,14 @@ key_list:
     ;
 
 key:
-    maybe_unary_operator PERCENTAGE { $$.id = 0; $$.isInt = false; $$.fValue = $1 * $2; $$.unit = CSSPrimitiveValue::CSS_NUMBER; }
+    maybe_unary_operator PERCENTAGE {
+        $$.id = 0; $$.isInt = false;
+        if ($1 > 0 && $2 <= 100) {
+            $$.fValue = $1 * $2; $$.unit = CSSPrimitiveValue::CSS_NUMBER;
+        } else {
+            $$.fValue = 0; $$.unit = CSSPrimitiveValue::CSS_UNKNOWN;
+        }
+    }
     | IDENT {
         $$.id = 0; $$.isInt = false; $$.unit = CSSPrimitiveValue::CSS_NUMBER;
         CSSParserString& str = $1;